One of the various fields of application of polytetrafluoroethylene [PTFE] moldings includes pumps, bellows, diaphragms and like apparatus members or elements required to have remarkable flexing resistance or flexibility.
Generally, compression molding techniques are frequently used in the art for obtaining those PTFE moldings. For compression molding of PTFE, there are available the batch type molding method comprising filling a mold with a resin powder, compressing the powder to form a preshaped compact mass and sintering the same, and the ram extrusion molding method comprising charging an axially long mold with a resin powder and continuously performing compression and sintering.
For the purpose of improving the flexing resistance of PTFE moldings, a technique is known which comprises lowering the crystallinity in moldings. For example, a sputter-resistant polytetrafluoroethylene tube intended for use as a mobile part or member and having a ratio between the strength in the longitudinal direction and the strength in the diameter direction of 2.5:1 to 1:1 has been proposed (cf. e.g. Patent Document 1: Japanese Kokai (Laid-open) Publication H11-70558). However, this technology uses a fine powder obtained by emulsion polymerization and the molding method applicable thereto is disadvantageously limited.
As a seal ring made of PTFE reduced in crystallinity, one whose crystallinity is controlled within the range of 25 to 35% has been proposed for the purpose of durability improvement (cf. e.g. Patent Document 2: Japanese Kokai Publication 2001-304420).
On the other hand, the crystallinity of PTFE moldings greatly influences the gas/chemical liquid barrier properties and, for improving the barrier properties, it is necessary to increase the crystallinity.
For producing high-crystallinity PTFE moldings, a method has been proposed which comprises sintering PTFE moldings at a temperature higher than the melting point of the resin, followed by cooling over 0.5 to 10 minutes at temperatures around the crystallization temperature (cf. e.g. Patent Document 3: Japanese Kokai Publication H06-8344). This method is to give porous bodies. Since it is necessary to carry out the cooling after sintering gradually, much time is required for the sintering and cooling steps, hence there is a productivity problem.
When PTFE moldings are obtained by compression molding in a narrow sense of the term and where flexing resistance is required in the intended use thereof, rapid cooling in the step of sintering preshaped bodies (preforms) is a general practice for attaining decreases in crystallinity (cf. e.g. Patent Document 1).
However, even when an attempt is made at rapid cooling in a sintering furnace, there arises a problem that insufficient rapid cooling will result due to the influence of the remaining heat. According to the method comprising taking the moldings in a molten state out of the furnace and immediately placing the same in water, it is possible to attain rapid cooling relatively easily but the method has workability and operability problems.
For compression molding in a narrow sense of the term, a method (hot coining) is also known which comprises removing the push punch alone while the preshaped preform is retained in the mold, heating the preform in a sintering furnace until melting of PTFE, taking the molding in a molten state, together with the mold, out of the sintering furnace, keeping the molding in a molten state again under pressure and cooling the same with water. This method makes it easy to reduce the crystallinity relatively easily and obtain PTFE moldings excellent in flexing resistance. However, this method has a problem in that the productivity is poor.
On the contrary, ram extrusion molding is generally excellent in productivity. The PTFE moldings obtained by ram extrusion molding are low in crystallinity owing to the method of production thereof. However, the method has a problem in that the moldings are poor in mechanical physical properties as compared with compression molding in a narrow sense of the term or with hot coining. Therefore, the tensile strength and tensile elongation are low and, in some cases, the joints or seams exert a very bad influence; therefore, under the existing circumstances, PTFE moldings obtained by ram extrusion molding are generally not used as important parts.
A powder proposed as a molding powder suited for compression molding in a narrow sense of the term and for ram extrusion molding is a powder of a modified PTFE species containing a specified amount of a specific perfluorovinyl ether unit species and having a specified heat of crystallization and having a specified specific surface area and a specified average particle diameter and, further, capable of providing moldings having a specified flex life and a specified level of creep resistance (cf. e.g. Patent Document 4: WO 93/16126). In recent years, however, moldings have been required to have still higher levels of physical properties.